Excluding unreleased material, it doesn't get much more obscure than "The Mess," a live track recorded in 1972 and released as the B-side to "My Love" in March 1973.

It's a danceable ode (as the video below proves) that probably started out as several different song ideas that got grafted together in typical McCartney fashion ("The Pound Is Sinking" from Tug of War is another example of the McCartney patchwork method).

"The Mess" was originally meant to be included on Wings' Red Rose Speedway album (It was supposed to be a double album at one point), and there's even a studio version of the song out there somewhere.